A wavelength-converting device is an optical transducing device, which is mainly utilized for converting a wavelength of light into a wavelength of visible light as a light source. Usually, it is applied to special lightings, such as spotlights, headlights, monitor lightings or projector lightings.
In general, most of conventional wavelength-converting devices are phosphor wheels. A kind of phosphor wheel is used for transforming laser light into color light with different wavelengths. Under the high-power operation, if a phosphor wheel has high optical converting efficiency, the photoelectric conversion can significantly enhance the luminance output of a projector. Accordingly, the phosphor wheel becomes an important light source of new generation projection technology in recent years.
Please refer to FIG. 1. FIG. 1 schematically illustrates the cross-sectional view of a conventional phosphor wheel of prior art. The conventional phosphor wheel 1 is a three-layer structure. The conventional phosphor wheel 1 has a substrate 10, a reflection layer 11 and a phosphor layer 12. The reflection layer 11 is formed on the substrate 10, and the phosphor layer 12 is formed on the reflection layer 11. That is, the reflection layer 11 is formed between the substrate 10 and the phosphor layer 12. After the first waveband light L1 is to excite the phosphor powder 121 in the phosphor layer 12 and transformed into second waveband light L2, the second waveband light L2 is omnidirectional emitted and scattered. As the emitting or scattering direction away from the reflection layer 11 in the phosphor layer 12 is defined as a backward direction, the emitting or scattering direction toward the reflection layer 11 in the phosphor layer 12 is called forward direction. The emitting or scattering light in the forward direction will be reflected by the reflection layer 11 and outputted in the backward direction. Because the second waveband light L2 transformed by the phosphor powder belongs to Lambertian reflectance model, the reflection layer 11 has to have the ability to reflect an incident light with an incidence angle greater than 70 degrees besides reflecting visible light with wavelength between 400-700 nanometers. It is a hard work and a difficult task to cope with the broad reflection band and the large incidence angle for the multilayer reflection mirror technology.
In addition, considering the Brewster angle (θB=tan−1(n2/n1)) effect of the incident environment n1 and the transmissive environment n2, when the incidence angle of an incident light is greater than or equal to the Brewster angle, the P-polarized light of the incident light will be fully transmitted through the reflection layer 11, such that the reflection rate of the reflection layer 11 is significantly decreased, which causing the light leakage phenomenon. For example, when incident light is transmitted into the air with refraction coefficient n=1 from a substance with an effective refraction coefficient n=1.4-1.5, the Brewster angle is 35.5 degrees, and a critical angle (θC=sin−1(n2/n1)) is 45.6 degrees. That is, when the incidence angle of the incident light is greater than or equal to 35.5 degrees and less than 45.6 degrees, the P-polarized light of the incident light will be fully transmitted through the reflection layer 11, and the light leakage phenomenon occurs. When the incidence angle of the incident light is greater than or equal to 45.6 degrees, the incident light will be fully reflected by the critical angle. It can be extrapolated that in the structure of the conventional phosphor wheel 1, since the reflection layer 11 is formed between the phosphor layer 12 (n1˜1.4-1.5) and the substrate 10 (ns) and the Brewster angle is less than the critical angle, when the incidence angle of the incident light is greater than or equal to the Brewster angle and less than the critical angle, there will be a lot of loss of the incident light, which cannot be reflected and applied to the optical path. A large amount of energy is wasted, and the difficulty of fabrication of the wavelength-converting device and the illumination system is significantly increased.
There is a need of providing a wavelength-converting device and an illumination system using the same to obviate the drawbacks encountered from the prior art.